Separatist tensions within the European Union looked set to grow with a surge of support for pro-independence parties at regional elections held on Sunday in Spain's northern Basque country.

A separatist coalition, EH Bildu, became the region's second biggest political group, taking a quarter of the vote as it reaped a peace dividend following armed separatist group Eta's decision to end four decades of terrorism last year.

The coalition brings together peaceful separatists with former members of Eta's political wing who are no longer banned from parliamentary politics. It won 21 of the 75 seats in the regional parliament.

The moderate Basque Nationalist party (PNV), which backs a gradual march towards an as yet undefined form of independence, lost votes to EH Bildu but was the overall winner with a third of the votes and 27 seats.

PNV looked set to form a minority government, possibly supported by EH Bildu, though party leader Iñigo Urkullu has refused to say exactly how he plans to govern. The two parties jointly won almost two-thirds of the seats, bringing an end to three years of socialist government under outgoing Basque prime minister Patxi López.

Although Urkullu has pledged to make the economy his first priority, he was widely expected to follow the example of Catalan regional president Artur Mas and call some kind of referendum on independence over the coming four years.

But it remained unclear whether his party would ask for full independence, a new statute of autonomy giving it greater self-government or simply demand that Spain explicitly allow it a right to self-determination in the future.

During the campaign Urkullu has both described his party as being "in favour of 21st century-style independence" and wanting a new "bilateral" relationship with Spain. Analysts say that reflects a party split between separatists and those seeking greater autonomy within Spain.

"The Spanish state has become a brake on the development of the Basque country," the PNV states in its manifesto.

Urkullu said that the absence of Eta meant these were the first properly free elections for decades. "This country cannot be built through confrontation or exclusion," he said.

He gave few clues, however, about his plans beyond saying that the Basque country must remain part of the EU.

The Basque election result will add to the problems of Spanish prime minister Mariano Rajoy, whose conservative People's party (PP) government has already vowed to block any independence referendums called by regional governments.

The wealthy and populous north-eastern Spanish region of Catalonia goes to the polls next month as support for independence there rises above 50% in opinion polls. Regional Catalan prime minister Artur Mas, whose nationalist Convergence and Union (CiU) coalition looks set to increase its seats in the regional parliament, has pledged to call a referendum despite Rajoy's threat to declare it illegal.

Catalan and Basque separatists are watching events in Scotland, hoping that a yes vote at an independence referendum there in 2014 would force the EU to come up with a formula allowing for member states to split up.

In separate elections held in Galicia in the north-west of Spain, Rajoy's People's party hung on to the regional government, despite the recession and 25% unemployment throughout the country.